Method of electrostatic filtration

ABSTRACT

Electrostatic filtration of secondary ions of mass m in a given mass ratio with a primary ion of mass M which has formed the secondary ions by fission is carried out by a method which consists in forming a singly-charged primary ion of the substance having a molecular mass M and extracting the ion at a voltage V1 with respect to ground, in causing the primary ion to cross a potential barrier V2, in producing the dissociation of said ion into at least two fragments of secondary ions, in extracting the fragment ion of mass m at a voltage V2, in carrying out a filtration in an electrostatic analyzer through which only the ions of energy eV&#39;&#39;&#39;&#39; are permitted to pass, in detecting the ions which have thus been filtered and the mass m of which is such that   D R A W I N G

United States Patent Devienne Oct. 28, 1975 METHOD OF ELECTROSTATIC Primary Examiner-James W. Lawrence FILTRATION Assistant Examiner-C. E. Church [75] Inventor: Fernand Marcel Devienne, Cannes Attorney Agent or Flrm cameron Kerkam Sutton F Stowell & Stowell rance [73] Assignee: Agence Nationale de Valorisation de la Recherche Anvar, [57] ABSTRACT Neuilly-sur-Seine, France Electrostatic filtration of secondary ions of mass m in [22] Filed Jan 18 1974 a given mass ratio with a primary ion of mass M which Appl. No.: 434,695

has formed the secondary ions by fission is carried out by a method which consists in forming a singlycharged primary ion of the substance having a molecular mass M and extracting the ion at a voltage V with respect to ground, in causing the primary ion to cross a potential barrier V in producing the dissociation of said ion into at least two fragments of secondary ions, in extracting the fragment ion of mass m at a voltage V in carrying out a filtration in an electrostatic analyzer through which only the ions of energy eV are permitted to pass, in detecting the ions which have thus been filtered and the mass m of which is such that I U.S. Patent Oct. 28, 1975 METHOD OF ELECTROSTATIC FILTRATION This invention relates to a method and a device for electrostatic filtration of secondary ions of mass m, said ions being in a given mass ratio with a primary ion of mass M which has formed said secondary ions by fission.

The invention has a large number of practical applications, especially in the field of accurate determination of the molecular mass of substances which have a high molecular mass, in the analysis of gas mixtures and in the separation of isotopes of a given chemical species.

The method of electrostatic filtration of ions in accordance with the invention is primarily characterized in that the following operations are carried out in succession:

formation of a singly-charged ion of the substance having a molecular mass M and extraction of said ion at a voltage V with respect to ground, thus conferring the kinetic energy eV on said ion;

crossing of a potential barrier V, by said ion which is thus given the kinetic energy e(V, V

dissociation of said primary ion into at least two fragments of secondary ions including one singlycharged fragment of mass m which consequently acquires the kinetic energy e(V V m/M;

extraction of said secondary-ion fragment of mass m at a voltage V,, thus bringing its kinetic energy to the value e(V V m/M eV filtration in an electrostatic analyzer which permits the passage only of those ions whose energy has a value eV";

detection of the ions which have thus been filtered and the mass m of which is such that The remarkable results obtained in many different fields by the application of the method according to the invention arise essentially from the fact that the primary ion of mass M in which it is desired to filter a secondary fragment of mass m is brought to a known kinetic energy before the secondary ion of mass m is separated therefrom and removes a part of the initial kinetic energy which is precisely in the ratio m/M of the masses which are present. It is accordingly apparent that, by carrying out an energy filtration by means of an electrostatic analyzer of a type known per se, the above-mentioned secondary-ion fragment of mass m can thus be filtered with great ease. Since in addition the ratio of masses m and M is expressed by a ratio of voltages which, in the present state of knowledge, can be measured without any difficulty and with very high accuracies up to for example, the method according to the invention therefore makes it possible to measure the ratio M/m with a standard of accuracy of the same order of magnitude. On the basis of the foregoing observation, a number of applications can be contemplated such as, for example, the following:

If the mass M of the primary ion is unknown, it will of interest especially if the mass M is of very high value and one of the products of dissociation of the primary ion M results in a well-known secondary ion of unit mass such as carbon C, for example, or one of the groups CH, N, CH OH, NH, 0, CH and so forth which are in that case readily identifiable by means of a low-resolution magnetic analyzer of the permanent magnet type.

If it is desired to detect the presence of a substance having a molecular mass M in a mixture of substances and especially a gas mixture, the method according to the invention provides a simple means of achieving this aim by selecting a readily identifiable secondary ion of mass m from the secondary ions which can be formed in a predictable manner at the time of dissociation of a primary ion of mass M. Under these conditions, it is only necessary to regulate the different voltages V V and V" employed in the method in order to form an ion filter which will allow only the intended secondary ion of mass m to pass out at the exit end of the electrostatic analyzer. The mere detection of the presence of this ion of mass m is accordingly sufficient to confirm that the compound of molecular mass M was present in the original mixture of substances and, if it is desired to determine in addition the relative proportion of said compound of molecular mass M in the original mixture, the intensity of the current of secondary ions of mass m which have thus been filtered makes it possible to obtain this information. This particular application of the method according to the invention is of practical interest in very important fields, especially in the monitoring of atmospheric air pollution and permits both identification and very rapid quantitative analysis of the different wellknown air pollutants in urban areas such as, for example, the oxygenated compounds of nitrogen and carbon.

The separation of isotopes of a given chemical substance can readily be carried out by means of the method according to the invention, starting from a compound of said substance. In fact, if molecules of said compound are ionized and if it is possible as a result of inelastic collision with neutral gas molecules to dissociate said ions in order that the isotope of mass m to be isolated may thus be caused to appear, it is then only necessary to adjust the voltages V V and V" in such a manner as to achieve filtration of said mass m of the isotope with respect to the molecular mass M of the starting compound. This is the case in particular with the uranium isotopes in which the hexafluoride UF of molecular mass M can produce by dissociation uranium ions having a mass m 235 which can thus readily be filtered by means of the method.

The present invention is also directed to a device for the practical application of the method described in the foregoing. Said device is mainly characterized in that it comprises in combination a source of ions of the substance of molecular mass M which are extracted at the voltage V,, a collision box containing molecules of gas and especially a rare gas and brought to a potential V an electrostatic analyzer for selecting the ions of energy eV", a retractable electrostatic detector for displaying the presence of secondary ions at the exit end of said analyzer.

In practice, the method according to the invention is performed very easily be means of equipment which is essentially of a conventional type. The main feature of the device employed lies in the fact that it comprises a collision box which contains molecules of gas and preferably of rare gases at a very low pressure, said collision box being brought to a predetermined potential V It is in this very box that the vital phase of the process takes place, namely the dissociation of the primary ions of mass M, the creation of secondary ions of mass in and the transfer to said secondary ions of the portion m/M of the initial energy of the primary ions of mass M. It is readily apparent that the application of the relations between mass and voltage which have been given earlier presupposes that the primary ion of mass M on the one hand and the secondary ion of mass m on the other hand are both carriers of a single electric charge. Nevertheless, if this were not the case, the method according to the invention still applies but the calculation is a little more complicated and the relations given above would be modified; it is for this reason that in practice preference is always given to the use of ions which carry a unit charge.

The detector which immediately follows the electrostatic analyzer meets two essential objectives: in the first place, it serves to check whether the ions of mass m have in fact been filtered and have passed through the system; in the second place, by measuring the peak height corresponding to the current of ions of mass m, the detector serves to determine the intensity of the filtered ion beam, which gives the value of the concentration of the ion M within the collision box.

Said detector is preferably intended to be removable in order that it can be withdrawn so as to permit inspection of the beam of ions of mass m in a magnetic analyzer. Should it be desired in fact to determine with certainty the exact molecular mass and therefore the nature of the secondary ion of mass m, there is in that case placed in position in accordance with the invention a magnetic analyzer having low resolution since all the ions have already been filtered in energy by the device; this analyzer can be of a simple permanent-magnet type which is low in cost price.

In accordance with an important feature of the device which is contemplated by the present invention, the ion source which serves to produce the primary ions of molecular mass M is itself constituted by a collision box of a type known per se which is brought to a potential V and in which the substance of molecular mass M in any desired form (solid, liquid or gaseous) is subjected directly to a molecular beam which is preferably a neutral gas and causes the desired ionization. In an improved alternative design of said device, the above-mentioned collision box is provided in addition to the aperture through which the molecular beam passes with one or a number of further lateral extraction apertures which serve to perform certain additional simultaneous analyses in regard to the characteristics of the formed ions of mass M. It will in any case be readily understood that any ion source of a type known per se, especially if it produces a beam having low energy dispersion, can be employed without thereby departing from the scope of the invention.

In another advantageous alternative embodiment of the present invention, the device aforesaid can comprise a plurality of collision boxes which are mounted in series and between each of which are placed an electrostatic analyzer and a removable detector. This arrangement indeed proves to be advantageous when the primary ion of molecular mass M to be analyzed has a very high mass and when it is necessary to carry out a number of successive decompositions in order to obtain as an end result an ion of mass m which can readily 4 be analyzed. In this case, the different successive collision boxes are brought to potentials V V V and the electrostatic analyzers are so adjusted as to filter the energies eV, eV eV. The relations between mass ratios and voltages must clearly be written at each dissociation stage.

In accordance with yet another feature of the inven tion, the voltages V V and V applied to the different components of the device aforesaid can be selected in accordance with a wide range of different modes ofoperation; said voltages may in particular be positive, negative or zero, constant or variable, depending on the particular problems to which the method of ion filtration in accordance with the invention is applied.

Compared with the conventional techniques of separation by mass spectrometry, the method according to the invention offers a large number of decisive advantages and chief among these can be listed the following:

the high standard of precision, the limit of which is solely a function of the performances of measure ment of a voltage ratio; the possibility of operating over ranges of molecular masses which overlap such as, for exmaple, 100,000 to 3,000, 5,000 to 200, 3,000 to 12 and 500 to l;

the separative power is independent of the peak ratio, that is to say of the relative concentrations of the substances which are present;

the possibility of readily separating two atoms which are chemically identical but are derived from the decomposition of two substances having only slightly different masses;

the possibility of readily separating compounds having very closely related masses such as, for exampl 12C 180 d 14N 16c)7 12C d 14N 1602- A clearer understanding of the invention will in any case be gained from the following description of two examples of operation of the ion filtration device, and subsequent examples of application of said device. The description is given without any implied limitation and makes reference to the accompanying drawings, in

which: t

FIG. 1 illustrates a device for the electrostatic filtration of ions by means of a single collision box;

FIG. 2 illustrates a device for the electrostatic filtration of ions which comprises two collision boxes.

There is shown in FIG. 1 a first collision box 1 of known type which is brought to a potential V and is used for the production of primary ions of molecular mass M by bombardment of a target by means of the molecular beam 2. The box 1 is fitted with means (not shown) for the introduction of the substance to be studied and comprises a device 3 consisting of plates for extracting ions of mass M which are formed within the box 1. By means of this extracting device 3, the ions of unit electric charge and of mass M produced within the box 1 are extracted from this latter with the kinetic energy eV;. In the example of FIG. 1, the collision box 1 further comprises a lateral exit 4 which is also provided with an extraction device 5. Said device 5 serves to ob tain at 6 a beam of ions of mass M for experiments or analyses which it is desired to perform at the same time as the filtration of ions in accordance with the invention A second collision box 7, also of known type, which has an entrance aperture 8 and an exit aperture 9 is brought to a potential V by means which are known per se and shown diagrammatically at 10. Said second box is also filled with molecules of a rare gas (helium, argon, neon or krypton) at a very low pressure within the range of torr to 2 X l0 torr, for example. The exit aperture 9 is provided with an extraction device 1 1 fitted in the same manner as the device 3 with a certain number of plates which are brought to different poten tials. The device of FIG. 1 then comprises an electrostatic analyzer 12 in which the two circular cylindrical electrodes 13 and 14 are brought to potentials such that the complete assembly allows only those ions which have a kinetic energy eV" to be filtered along the circular median path 15. In the conventional manner, the electrodes 13 and 14 are brought to symmetrical potentials with respect to ground in order to ensure that the path 15 itself is at ground potential and that the ions which follow said path are not subjected within the interior of the analyzer 12 to any exchange of energy with the field as this would have the effect of either accelerating the ions or of slowing them down. The analyzer 12 can be of any known type and can in particular have any angular aperture which may be desired. In the example described, an angular aperture equal to 127 has been selected.

The detector 16 is mounted at the exit of the analyzer 12 and is retractable, with the result that it can be placed either in the position 16a shown in full lines in which case it intercepts the beam or in the withdrawn position 16b shown in chain-dotted lines in which case it does not intercept the beam.

The device of FIG. 1 is finally completed by a lowresolution magnetic analyzer 17 of the permanent-magnet type equipped with its detector 18. The magnetic analyzer 17 and the detector 18 of a type known per se are employed solely when it is desired to carry out accurate identification of the mass m and therefore of the chemical nature of the ions filtered by the electrostatic analyzer 12. In FIG. 1, the different elements are illustrated in a highly diagrammatic plan view seen from above; similarly it has been preferred for the sake of enhanced clarity of the drawings to omit from these latter all the systems for creating a vacuum within those parts of the installation which contribute to the circulation of the ions or the production of the molecular beam, arrangements of this type being in themselves sufficiently well-known to those versed in the art to dispense with any need to describe them.

The operation of the device of FIG. 1 is as follows: under the action of the impact of the molecular beam 2, ions of mass M are detached from the substance to be studied which has previously been introduced into the box 1 and are extracted with the kinetic energy eV from the collision box 1 along the path 19. Said ions in turn penetrate at 8 into the collision box 7 which is on the one hand brought to the potential V and on the other hand filled with a gas which is preferably a rare gas in order to prevent any possible chemical reactions between the molecules of said gas and the ions of molecular mass M. Within the collision box 7 a certain number of primary ions of mass M are caused to dissociate under the action of collisions which they are liable to undergo with the molecules of neutral gas. Under these conditions if, within the interior of the collision box 7, a primary ion of mass M on which is conferred a kinetic energy corresponding to the value e (V V dissociates and produces a secondary ion of mass m which carries the same unit electric charge as the ion M, the secondary ion of mass m carries away a fraction of the energy e (V V which is in the ratio m/M of the masses of the secondary ion and of the primary ion. Said secondary ion of mass m is then extracted by the plate device 11 at the voltage V which imparts to said ion a complementary kinetic energy eV Among all the possible secondary ions which are derived from the dissociation of the primary ion of mass M within the collision box 7, the electrostatic analyzer 12 makes it possible to select those which possess a given kinetic energy equal to eV". The detection and analysis of these secondary ions of mass m are then carried out according to the different potential applications by the retractable detector 16 and the magnetic analyzer 17. The essential feature of the device shown in FIG. 1 in any case lies in the fact that, by simply measuring a ratio of two voltages, said device makes it possible to determine with a very high degree of accuracy the ratio of the masses M and m corresponding respectively to the primary ion under study and to one of the secondary ions derived from its dissociation as a result of impact on the molecules contained in the collision box 7.

In the case of FIG. 2, the device is identical with the device of FIG. 1 as far as the analyzer 16 but then has a second stage which is identical with the first. This second stage is made up of a second collision box 20 which is brought to a potential V by means 21 known per se. The collision box 20 is fitted with a device 22 for the extraction of formed ions and is filled internally with a rare gas at low pressure in the same manner as the box 7. The box 20 is followed by a second electrostatic analyzer 23 which is similar to the analyzer 12; the electrostatic analyzer 23 is in turn completed by a retractable detector 24 and if necessary by a low-resolution magnetic analyzer 25 of the permanent-magnet type which is in turn followed by its detector 26.

In the case of FIG. 2, if V designates the potential of the box 20, m designates the mass of the tertiary ion formed by collision within said box and eV designates the filtration energy of the electrostatic analyzer 23, an elementary calculation shows that the value of the filtration ratio is given by the formula:

" 2) 0'' 3) I4" 0 m w" v.)

First example of application.

There will now be described one example of determination of a molecular mass by means of the method according to the invention. Let it be postulated by way of example that it is desired to analyze an unknown organic substance having the real formula C H 0 N P This substance has a molecular mass which is exactly equal to 1341.6164. Provided that the primary ions of mass M are extracted at a sufficiently high voltage V there are obtained as the most common fragments of dissociation in the decomposition of organic substances by inelastic collision: carbon, the CH group, the CH group, nitrogen, the NH group, the OH group, the C H group, the acetylene group C H and in some cases also phosphorus, sulphur and the SH group. In practice, it is a great advantage to select from the fragments of decomposition of mass m which are produced in the form of secondary ions within the collision box 7 light and readily identifiable elements and compounds such as carbon C, the OH group and the acetylene group C H In the case of particular values of V and V", there have been found the three following ratios each corresponding to one of the three light groups which have been set forth in the foregoing:

In the case of the secondary ion of mass m which is constituted by carbon, M/m 111.798.

In the case of the secondary ion of mass m which is constituted by the OH group, M/m 78.907.

In the case of the secondary ion of mass m which is constituted by the C H group, M/m 51.569.

This leads respectively in the case of the mass M to the three following values:

It is deduced from the foregoing that the unknown molecular mass M is equal on an average to 1341.605 i 0.03.

Second example of application.

It is again proposed to carry out the measurement of an unknown molecular mass. The device of FIG. 1 is employed in this example, wherein V has a constant value equal to 10,000 volts, V has a constant value equal to 100 volts and V is chosen so as to be variable and positive, in order to obtain carbon having a mass m 12. In order that the characteristic peak of the carbon ion should in fact be obtained in the magnetic analyzer 17, experience has shown that a voltage V 59.7609 volts must be applied to the collision box 7. It is deduced therefrom that Third example of measurement of molecular mass.

It is desired to determine the unknown molecular mass M of a primary ion which undergoes fragmentation and produces carbon of mass 12 as secondary ion. In the mode of operation which is employed for the device of FIG. 1, the selected values are V" 100 volts, V l volts and the variable positive potential V of the collision box 1 is adopted. It is found experimentally that in order to obtain the characteristic peak of carbon in the magnetic analyzer 17, the box 1 must be brought to the potential V 5605.167 volts. From this it is deduced that the unknown mass is This corresponds to lactose, the formula of which is 12 zz u- This example is of interest since it serves to demonstrate in a particular case that there cannot be any ambiguity between two molecular masses of very closely related value. In fact, postulating the existance of a substance having the formula C H O N whose mass is not 342.116 but 342.140, it is in that case necessary in order to filter the carbon ions of mass m 12 in the analyzer 12 to apply to the box 1 a voltage V which is equal in this instance to 5602.333 volts. It is apparent that in order to obtain the two carbon peaks relative to secondary ions derived from the dissociation of primary ions having masses M which are as little different from each other as 342.116 and 342.140, it is accordingly necessary to apply to the box 1 two voltages having the difference of 0.4 volt, such an order of magnitude being distinctly higher than possible errors of measurement.

8 Measurement of very high molecular masses (of the order of 10,000 or more).

In the case of very high molecular masses, it is particularly advantageous to employ the device of FIG. 2 in which provision is made for two collision boxes 7 and 20. It is only necessary in this case to adjust the different potentials in such a manner as to ensure, for example, that the filtration ratio M/m between the primary ions and the secondary ions in each of the boxes 7 and 20 is of the order of magnitude of 20 to 50. Under these conditions, molecular masses of the order of 10,000 can clearly be determined with accuracy. It is even feasible to exceed this order of magnitude and to measure molecular masses up to 200,000 by employing for example a device consisting of three collision boxes in series.

Application to the analysis of mixtures of substances.

One of the applications of the method according to the invention which is of particular interest consists in determining the existence of a compound, even in the state of traces, in a predetermined mixture of substances. It is even more especially worthy of note that the invention meets a real need in the determination of atmospheric air pollution in cities and surrounding areas or industrial centers by permitting simple and immediate determination of polluting compounds even if they are present in the atmosphere in the state of traces. By way of example, one method of determination of nitric oxide NO in atmospheric air will be described hereinafter. It will first be recalled that this determination cannot readily be performed by conventional methods of mass spectrometry if the concentrations are of a low order and especially within the range of 10 to 10' for example. In point of fact, the peaks of nitrogen and oxygen in this case are of such high value that it is impossible to separate the peak of nitric oxide NO when the sensitivity is considerably increased as is clearly necessary at this very low level of concentration (the method is also made more difficult by the fact that the peak of carbon monoxide CO, the peak of nitric oxide NO and the peak of molecular nitrogen N practically coincide). In order to analyze possible air pollution from the oxide NO, the air is introduced into the box 1 and the molecules contained therein are ionized. Thus if the analyzed air in fact contains molecules of oxide NO, they are converted to NO ions which are then dissociated into atomic ions N+ and 0+; the problem is thus very readily solved since it is impossible to confuse the atomic ion N+ derived from the dissociation of nitric oxide NO with the nitrogen ion N+ derived from the decomposition of the molecule of nitrogen N In fact, the precise atomic mass of nitric oxide NO is 29.9979. If the filtration of an atomic ion of nitrogen is achieved in the device of FIG. 1, the ratio m/M is equal to: l4.003074/29.9979 0.4668 when said nitrogen is derived from the dissociation of nitric oxide NO whereas, on the contrary, a ratio of 0.5 is obtained when the dissociation of a molecular ion N of free nitrogen is involved.

In the specific example, the device of FIG. 1 is employed by adopting a constant negative value equal to 500 volts in the case of V and a value of 50 volts in the case of V whereas V is variable. Under these conditions, if the characteristic peak of nitrogen is in fact detected in the magnetic analyzer 17, two cases can accordingly arise:

thus resulting in a voltage V 678.21 volts.

2. Atomic nitrogen N+ derived from the decomposition of a molecular ion N in thiscase, the ratio where V 600 V.

It is therefore apparent that there is no possible ambiguity and that it is a very easy matter by means of this method to distinguish the case of a nitrogen atom derived from atmospheric nitrogen from the case of a nitrogen atom derived from the dissociation of a molecule of oxide NO, which is strictly impossible in conventional mass spectrometry. In practice, if it is desired to monitor the appearance of nitric oxide NO in air, the potential of the box V, is fixed at the value of 678.21 volts and any signal at the electrostatic detector 16 can be interpreted as indicating the actual presence of N molecules in the air under analysis.

Finally, the observation of the dimension of the peak of nitrogen in the detector 16 makes it possible in the above-mentioned case No l to have a measurement of the proportion of nitric oxide NO which is contained in the air to be monitored.

What we claim is: l. A method of electrostatic filtration of secondary ions of mass m, said ions being in a given mass ratio with a primary ion of mass M which has formed said secondary ions by fission, wherein the following operations are carried out in succession:

formation of a singly-charged primary ion of the substance having a molecular mass M and extraction of said ion at a voltage V, with respect to ground, thus conferring the ltineticenergy eV, on said ion;

crossing of a potential barrier V, by said ion which is thus given the kinetic energy e(V V,);

dissociation of said primary ion into at least two fragments of secondary ions including one singlycharged fragment of mass m which consequently acquires the kinetic energy e(V, V m/M;

extraction of said secondary ion fragment of mass m at a voltage V,, thus bringing its kinetic energy to the value e(V, V,) m/M eV,;

filtration in an electrostatic analyzer which permits the passage only of those ions whose energy has a value'eV";

detection of the ions which have thus been filtered and the mass m of which is such that Z. A method according to claim I, wherein the primary ions of mass M are produced by'an ion source 10 lecular beam with the substance of molecular mass M to be ionized.

4. A method according to claim 1, wherein the dissociation of the primary ions occurs by inelastic impact of said ions on gas molecules and especially molecules of rare gases.

5. A method according to claim 1, wherein the detection of the dissociated ion fragment of mass m at the exit of the electrostatic analyzer is carried out by means of a detector.

6. A method according to claim 1, wherein the identification of the secondary ion of mass m is carried out by means of a low-resolution magnetic analyzer of the permanent magnet type.

7. A method according to claim 1 wherein, in order to filter very high molecular masses, a number of successive fragmentations in series of the primary ion of mass M are carried out at potentials V V V,,, each fragmentation being followed by a filtration of energy eV", eV' eV" in an electrostatic analyzer.

8. Application of the method according to claim 1 to the determination of the unknown molecular mass M of a primary ion, wherein the mass m of the filtered secondary ion is determined and the mass M is calculated by means of the relation 1 M v" v,

9. Application of the method according to claim I to the detection of a known substance having a given mass M in a mixture of substances, wherein the voltages V,, V, and V" are so adjusted as to form a filter for one of the secondary ions of mass m which are expected in the dissociation of the primary ion of mass M and wherein the actual presence of said secondary ion is detected at the exit of the electrostatic analyzer.

10. Application of the method according to claim I to the separation of isotopes of a chemical substance, wherein the voltages V,, V, and V" are so adjusted as to form a filter for one of the isotopes of mass m to be isolated, starting from the expected products of dissociation of the primary ion of mass M.

11. A device according to claim 10, wherein provision is additionally made for a low-resolution magnetic analyzer of the permanent magnet type.

12. A device for carrying out the method according to claim 1, wherein said device comprises in combination a source of ions of the substance of molecular mass M which are extracted at the voltage V a collision box containing molecules of a rare gas and brought to a potential V an electrostatic analyzer for selecting the ions of energy eV", a retractable electrostatic detector for displaying the presence of secondary ions at the exit of said analyzer.

13. A device according to claim 12, wherein the ion source is constituted by a collision box containing the substance of molecular mass M and brought to a potential V,, a molecular beam being applied to the entrance aperture of said collision box.

14. A device according to claim 13, wherein the collision box is further provided with one or a number of lateral extraction apertures so as to permit one or a number of additional simultaneous analyses. 

1. A method of electrostatic filtration of secondary ions of mass m, said ions being in a given mass ratio with a primary ion of mass M which has formed said secondary ions by fission, wherein the following operations are carried out in succession: formation of a singly-charged primary ion of the substance having a molecular mass M and extraction of said ion at a voltage V1 with respect to ground, thus conferring the kinetic energy eV1 on said ion; crossing of a potential barrier V2 by said ion which is thus given the kinetic energy e(V1 - V2); dissociation of said primary ion into at least two fragments of secondary ions including one singlycharged fragment of mass m which consequently acquires the kinetic energy e(V1 - V2) m/M; extraction of said secondary ion fragment of mass m at a voltage V2, thus bringing its kinetic energy to the value e(V1 - V2) m/M + eV2; filtration in an electrostatic analyzer which permits the passage only of those ions whose energy has a value eV''''; detection of the ions which have thus been filtered and the mass m of which is such that
 2. A method according to claim 1, wherein the primary ions of mass M are produced by an ion source having a low energy dispersion.
 3. A method according to claim 1, wherein the primary ions of mass M are produced by collision of a molecular beam with the substance of molecular mass M to be ionized.
 4. A method according to claim 1, wherein the dissociation of the primary ions occurs by inelastic impact of said ions on gas molecules and especially molecules of rare gases.
 5. A method according to claim 1, wherein the detection of the dissociated ion fragment of mass m at the exit of the electrostatic analyzer is carried out by means of a detector.
 6. A method according to claim 1, wherein the identification of the secondary ion of mass m is carried out by means of a low-resolution magnetic analyzer of the permanent magnet type.
 7. A method according to claim 1 wherein, in order to filter very high molecular masses, a number of successive fragmentations in series of the primary ion of mass M are carried out at potentials V2, V3 . . . Vn, each fragmentation being followed by a filtration of energy eV'''', eV'''''' . . . eVn in an electrostatic analyzer.
 8. Application of the method according to claim 1 to the determination of the unknown molecular mass M of a primary ion, wherein the mass m of the filtered secondary ion is determined and the mass M is calculated by means of the relation
 9. Application of the method according to claim 1 to the detection of a known substance having a given mass M in a mixture of substances, wherein the voltages V1, V2 and V'''' are so adjusted as to form a filter for one of the secondary ions of mass m which are expected in the dissociation of the primary ion of mass M and wherein the actual presence of said secondary ion is detected at the exit of the electrostatic analyzer.
 10. Application of the method according to claim 1 to the separation of isotopes of a chemical substance, wherein the voltages V1, V2 and V'''' are so adjusted as to form a filter for one of the isotopes of mass m to be isolated, starting from the expected products of dissociation of the primary ion of mass M.
 11. A device according to claim 10, wherein provision is additionally made for a low-resolution magnetic analyzer of the permanent Magnet type.
 12. A device for carrying out the method according to claim 1, wherein said device comprises in combination a source of ions of the substance of molecular mass M which are extracted at the voltage V1, a collision box containing molecules of a rare gas and brought to a potential V2, an electrostatic analyzer for selecting the ions of energy eV'''', a retractable electrostatic detector for displaying the presence of secondary ions at the exit of said analyzer.
 13. A device according to claim 12, wherein the ion source is constituted by a collision box containing the substance of molecular mass M and brought to a potential V1, a molecular beam being applied to the entrance aperture of said collision box.
 14. A device according to claim 13, wherein the collision box is further provided with one or a number of lateral extraction apertures so as to permit one or a number of additional simultaneous analyses. 